1. Field of the Invention
The present invention relates to a MRI (magnetic resonance imaging) apparatus and a magnetic resonance imaging method which excite nuclear spin of an object magnetically with a RF (radio frequency) signal having the Larmor frequency and reconstruct an image based on NMR (nuclear magnetic resonance) signals generated due to the excitation, and more particularly, to a magnetic resonance imaging apparatus and a magnetic resonance imaging method which make it possible to perform non-contrast enhanced MRA (Magnetic Resonance Angiography) to obtain a blood flow image without contrast medium.
2. Description of the Related Art
Magnetic Resonance Imaging is an imaging method which excites nuclear spin of an object set in a static magnetic field with a RF signal having the Larmor frequency magnetically and reconstruct an image based on NMR signals generated due to the excitation.
In the field of the magnetic resonance imaging, as a method of obtaining an image of a blood flow, MRA is known. An MRI that uses a contrast medium is referred to as a contrast MRA. By the dynamic contrast enhanced MRA, an breast imaging and differentiation of a tumor depicted in a breast have been conventionally performed (see, for example, Kuhl, CK, Radiology 1999; 211:101-110 and Kuhl, CK Radiology: Volume 244: Number 2-August 2007).
Specifically, time series DCE (dynamic contrast enhanced) images of a breast are acquired by a dynamic imaging of an object into which a contrast medium is injected in advance, and a perfusion curve which indicates the contrast-enhanced effect by the contrast medium, i.e., a temporal variation in the degree of dyeing the breast by the contrast medium, is generated from the DCE images. Then, an examination on whether a tumor imaged in the breast is benign or malignant is performed by examining time variations of a wash-in part and a wash-out part of the contrast medium in the perfusion curve.
However, a blood flow image can not be acquired with a sufficient contrast without acquiring the first point dyed by a contrast medium as a contrast-enhanced effect within 60 seconds after injection of a contrast medium under the conventional method for differentiating a tumor in a breast based on DCE images since the first time phase in which a blood flow image can be acquired with a sufficient contrast is in a wash-in part within about 60 seconds after injection of contrast medium. Consequently, a time resolution is restricted. Further, a spatial resolution is reduced even by a high speed imaging due to the restriction in a time resolution. As a result, imaging a tumor of under 5 mm is difficult in the conventional technique.
Moreover, though the conventional examination of a tumor based on DCE images requires injection of a Gd type of contrast medium, it has recently been pointed out that there is an association between a Gd type of contrast medium and the NSF (Nephrogenic Systemic Fibrosis).